Pile Foundations Introduction

Transcription

1 11 Pile Foundations 11.1 Introduction Piles are structural members that are made of steel, concrete, or timber. They are used to build pile foundations, which are deep and which cost more than shallow foundations. (See Chapters 3, 4, and 5.) Despite the cost, the use of piles often is necessary to ensure structural safety. The following list identifies some of the conditions that require pile foundations (Vesic, 1977): 1. When one or more upper soil layers are highly compressible and too weak to support the load transmitted by the superstructure, piles are used to transmit the load to underlying bedrock or a stronger soil layer, as shown in Figure 11.1a. When bedrock is not encountered at a reasonable depth below the ground surface, piles are used to transmit the structural load to the soil gradually. The resistance to the applied structural load is derived mainly from the frictional resistance developed at the soil pile interface. (See Figure 11.1b.) 2. When subjected to horizontal forces (see Figure 11.1c), pile foundations resist by bending, while still supporting the vertical load transmitted by the superstructure. This type of situation is generally encountered in the design and construction of earth-retaining structures and foundations of tall structures that are subjected to high wind or to earthquake forces. 3. In many cases, expansive and collapsible soils may be present at the site of a proposed structure. These soils may extend to a great depth below the ground surface. Expansive soils swell and shrink as their moisture content increases and decreases, and the pressure of the swelling can be considerable. If shallow foundations are used in such circumstances, the structure may suffer considerable damage. However, pile foundations may be considered as an alternative when piles are extended beyond the active zone, which is where swelling and shrinking occur. (See Figure 11.1d) Soils such as loess are collapsible in nature. When the moisture content of these soils increases, their structures may break down. A sudden decrease in the void ratio of soil induces large settlements of structures supported by shallow foundations. In such 535

2 536 Chapter 11: Pile Foundations (a) Rock (b) (c) Zone of erosion Swelling soil Stable soil (d) (e) (f) Figure 11.1 Conditions that require the use of pile foundations cases, pile foundations may be used in which the piles are extended into stable soil layers beyond the zone where moisture will change. 4. The foundations of some structures, such as transmission towers, offshore platforms, and basement mats below the water table, are subjected to uplifting forces. Piles are sometimes used for these foundations to resist the uplifting force. (See Figure 11.1e.) 5. Bridge abutments and piers are usually constructed over pile foundations to avoid the loss of bearing capacity that a shallow foundation might suffer because of soil erosion at the ground surface. (See Figure 11.1f.) Although numerous investigations, both theoretical and experimental, have been conducted in the past to predict the behavior and the load-bearing capacity of piles in granular and cohesive soils, the mechanisms are not yet entirely understood and may never be. The design and analysis of pile foundations may thus be considered somewhat of an art as a result of the uncertainties involved in working with some subsoil conditions. This chapter discusses the present state of the art.

3 11.2 Types of Piles and Their Structural Characteristics Types of Piles and Their Structural Characteristics Different types of piles are used in construction work, depending on the type of load to be carried, the subsoil conditions, and the location of the water table. Piles can be divided into the following categories: (a) steel piles, (b) concrete piles, (c) wooden (timber) piles, and (d) composite piles. Steel Piles Steel piles generally are either pipe piles or rolled steel H-section piles. Pipe piles can be driven into the ground with their ends open or closed. Wide-flange and I-section steel beams can also be used as piles. However, H-section piles are usually preferred because their web and flange thicknesses are equal. (In wide-flange and I-section beams, the web thicknesses are smaller than the thicknesses of the flange.) Table 11.1 gives the dimensions of some standard H-section steel piles used in the United States. Table 11.2 shows selected pipe sections frequency used for piling purposes. In many cases, the pipe piles are filled with concrete after they have been driven. The allowable structural capacity for steel piles is Q all 5 A s f s (11.1) where A s 5 cross-sectional area of the steel f s 5 allowable stress of steel ( < f y ) Once the design load for a pile is fixed, one should determine, on the basis of geotechnical considerations, whether Q (design) is within the allowable range as defined by Eq When necessary, steel piles are spliced by welding or by riveting. Figure 11.2a shows a typical splice by welding for an H-pile. A typical splice by welding for a pipe pile is shown in Figure 11.2b. Figure 11.2c is a diagram of a splice of an H-pile by rivets or bolts. When hard driving conditions are expected, such as driving through dense gravel, shale, or soft rock, steel piles can be fitted with driving points or shoes. Figures 11.2d and 11.2e are diagrams of two types of shoe used for pipe piles. Steel piles may be subject to corrosion. For example, swamps, peats, and other organic soils are corrosive. Soils that have a ph greater than 7 are not so corrosive. To offset the effect of corrosion, an additional thickness of steel (over the actual designed cross-sectional area) is generally recommended. In many circumstances factory-applied epoxy coatings on piles work satisfactorily against corrosion. These coatings are not easily damaged by pile driving. Concrete encasement of steel piles in most corrosive zones also protects against corrosion. Following are some general facts about steel piles: Usual length: 15 m to 60 m Usual load: 300 kn to 1200 kn

4 538 Chapter 11: Pile Foundations Advantages: a. Easy to handle with respect to cutoff and extension to the desired length b. Can stand high driving stresses c. Can penetrate hard layers such as dense gravel and soft rock d. High load-carrying capacity Disadvantages: a. Relatively costly b. High level of noise during pile driving c. Subject to corrosion d. H-piles may be damaged or deflected from the vertical during driving through hard layers or past major obstructions Table 11.1 Common H-Pile Sections used in the United States Moment of inertia Designation, Flange and web ( ) size (mm) Depth Section area thickness w Flange width m4 3 3 d d 2 weight (kg/m) (mm) ( m ) (mm) (mm) HP HP HP HP HP y l xx l yy d 2 d 1 x x w y w

5 11.2 Types of Piles and Their Structural Characteristics 539 Table 11.2 Selected Pipe Pile Sections Outside Wall Area of diameter thickness steel (mm) (mm) (cm 2 )

6 540 Chapter 11: Pile Foundations Weld Weld (a) (b) (c) Weld Weld (d) Figure 11.2 Steel piles: (a) splicing of H-pile by welding; (b) splicing of pipe pile by welding; (c) splicing of H-pile by rivets and bolts; (d) flat driving point of pipe pile; (e) conical driving point of pipe pile Concrete Piles Concrete piles may be divided into two basic categories: (a) precast piles and (b) cast-in-situ piles. Precast piles can be prepared by using ordinary reinforcement, and they can be square or octagonal in cross section. (See Figure 11.3.) Reinforcement is provided to enable the pile to resist the bending moment developed during pickup and transportation, the vertical load, and the bending moment caused by a lateral load. The piles are cast to desired lengths and cured before being transported to the work sites. Some general facts about concrete piles are as follows: Usual length: 10 m to 15 m Usual load: 300 kn to 3000 kn Advantages: a. Can be subjected to hard driving b. Corrosion resistant c. Can be easily combined with a concrete superstructure (e)

7 11.2 Types of Piles and Their Structural Characteristics 541 2D Square pile Octagonal pile D D Figure 11.3 Precast piles with ordinary reinforcement Disadvantages: a. Difficult to achieve proper cutoff b. Difficult to transport Precast piles can also be prestressed by the use of high-strength steel pre-stressing cables. The ultimate strength of these cables is about 1800 MN>m 2. During casting of the piles, the cables are pretensioned to about 900 to 1300 MN>m 2, and concrete is poured around them. After curing, the cables are cut, producing a compressive force on the pile section. Table 11.3 gives additional information about prestressed concrete piles with square and octagonal cross sections. Some general facts about precast prestressed piles are as follows: Usual length: 10 m to 45 m Maximum length: 60 m Maximum load: 7500 kn to 8500 kn The advantages and disadvantages are the same as those of precast piles. Cast-in-situ, or cast-in-place, piles are built by making a hole in the ground and then filling it with concrete. Various types of cast-in-place concrete piles are currently used in construction, and most of them have been patented by their manufacturers. These piles may be divided into two broad categories: (a) cased and (b) uncased. Both types may have a pedestal at the bottom. Cased piles are made by driving a steel casing into the ground with the help of a mandrel placed inside the casing. When the pile reaches the proper depth the mandrel is withdrawn and the casing is filled with concrete. Figures 11.4a, 11.4b, 11.4c, and 11.4d show some examples of cased piles without a pedestal. Figure 11.4e shows a cased pile with a pedestal. The pedestal is an expanded concrete bulb that is formed by dropping a hammer on fresh concrete. Some general facts about cased cast-in-place piles are as follows: Usual length: 5 m to 15 m Maximum length: 30 m to 40 m Usual load: 200 kn to 500 kn Approximate maximum load: 800 kn

8 542 Chapter 11: Pile Foundations Table 11.3 Typical Prestressed Concrete Pile in Use Design bearing capacity (kn) Strength Minimum Number of strands of concrete Area of cross effective Section (MN/m 2 ) Pile D section Perimeter 12.7-mm 11.1-mm prestress modulus shape a (mm) (cm 2 ) (mm) diameter diameter force (kn) (m ) S O S O S O S O S O S O S O S O a S 5 square section; O 5 octagonal section Prestressed strand Wire spiral D Wire spiral Prestressed strand D Advantages: a. Relatively cheap b. Allow for inspection before pouring concrete c. Easy to extend Disadvantages: a. Difficult to splice after concreting b. Thin casings may be damaged during driving Allowable load: Q all 5 A s f s 1 A c f c (11.2) where A s 5 area of cross section of steel A c 5 area of cross section of concrete f s 5 allowable stress of steel f c 5 allowable stress of concrete

9 11.2 Types of Piles and Their Structural Characteristics 543 Raymond Step-Taper Pile Corrugated thin cylindrical casing Maximum usual length: 30 m Monotube or Union Metal Pile Thin, fluted, tapered steel casing driven without mandrel Maximum usual length: 40 m Western Cased Pile Thin metal casing Maximum usual length: 30 m 40 m (a) (b) (c) Seamless Pile or Armco Pile Thin metal casing Maximum usual length: 30 m 40 m Franki Cased Pedestal Pile Straight steel pile casing Maximum usual length: 30 m 40 m (d) (e) Western Uncased Pile without Pedestal Maximum usual length: 15 m 20 m Franki Uncased Pedestal Pile Maximum usual length: 30 m 40 m (f) (g) Figure 11.4 Cast-in-place concrete piles

10 544 Chapter 11: Pile Foundations Figures 11.4f and 11.4g are two types of uncased pile, one with a pedestal and the other without. The uncased piles are made by first driving the casing to the desired depth and then filling it with fresh concrete. The casing is then gradually withdrawn. Following are some general facts about uncased cast-in-place concrete piles: Usual length: 5 m to 15 m Maximum length: 30 m to 40 m Usual load: 300 kn to 500 kn Approximate maximum load: 700 kn Advantages: a. Initially economical b. Can be finished at any elevation Disadvantages: a. Voids may be created if concrete is placed rapidly b. Difficult to splice after concreting c. In soft soils, the sides of the hole may cave in, squeezing the concrete Allowable load: where A c 5 area of cross section of concrete f c 5 allowable stress of concrete Q all 5 A c f c (11.3) Timber Piles Timber piles are tree trunks that have had their branches and bark carefully trimmed off. The maximum length of most timber piles is 10 to 20 m. To qualify for use as a pile, the timber should be straight, sound, and without any defects. The American Society of Civil Engineers Manual of Practice, No. 17 (1959), divided timber piles into three classes: 1. Class A piles carry heavy loads. The minimum diameter of the butt should be 356 mm. 2. Class B piles are used to carry medium loads. The minimum butt diameter should be 305 to 330 mm. 3. Class C piles are used in temporary construction work. They can be used permanently for structures when the entire pile is below the water table. The minimum butt diameter should be 305 mm. In any case, a pile tip should not have a diameter less than 150 mm. Timber piles cannot withstand hard driving stress; therefore, the pile capacity is generally limited. Steel shoes may be used to avoid damage at the pile tip (bottom). The tops of timber piles may also be damaged during the driving operation. The crushing of the wooden fibers caused by the impact of the hammer is referred to as brooming. To avoid damage to the top of the pile, a metal band or a cap may be used. Splicing of timber piles should be avoided, particularly when they are expected to carry a tensile load or a lateral load. However, if splicing is necessary, it can be done by using pipe sleeves (see Figure 11.5a) or metal straps and bolts (see Figure 11.5b). The length of the sleeve should be at least five times the diameter of the pile. The butting ends should be cut square so that full contact can be maintained. The spliced portions should be carefully trimmed so that they fit tightly to the inside of the pipe sleeve. In the case of metal straps and bolts, the butting ends should also be cut square. The sides of the spliced portion should be trimmed plane for putting the straps on.

11 11.2 Types of Piles and Their Structural Characteristics 545 Metal sleeve Ends cut square Metal strap Ends cut square Metal strap (a) (b) Figure 11.5 Splicing of timber piles: (a) use of pipe sleeves; (b) use of metal straps and bolts Timber piles can stay undamaged indefinitely if they are surrounded by saturated soil. However, in a marine environment, timber piles are subject to attack by various organisms and can be damaged extensively in a few months. When located above the water table, the piles are subject to attack by insects. The life of the piles may be increased by treating them with preservatives such as creosote. The allowable load-carrying capacity of wooden piles is Q all 5 A p f w (11.4) where A p 5 average area of cross section of the pile f w 5 allowable stress on the timber The following allowable stresses are for pressure-treated round timber piles made from Pacific Coast Douglas fir and Southern pine used in hydraulic structures (ASCE, 1993): Pacific Coast Douglas Fir Compression parallel to grain: 6.04 MN>m 2 Bending: 11.7 MN>m 2 Horizontal shear: 0.66 MN>m 2 Compression perpendicular to grain: 1.31 MN>m 2 Southern Pine Compression parallel to grain: 5.7 MN>m 2 Bending: 11.4 MN>m 2

12 546 Chapter 11: Pile Foundations Horizontal shear: 0.62 MN>m 2 Compression perpendicular to grain: 1.41 MN>m 2 The usual length of wooden piles is 5 m to 15 m. The maximum length is about 30 m to 40 m (100 ft to 130 ft). The usual load carried by wooden piles is 300 kn to 500 kn. Composite Piles The upper and lower portions of composite piles are made of different materials. For example, composite piles may be made of steel and concrete or timber and concrete. Steel-and-concrete piles consist of a lower portion of steel and an upper portion of castin-place concrete. This type of pile is used when the length of the pile required for adequate bearing exceeds the capacity of simple cast-in-place concrete piles. Timber-andconcrete piles usually consist of a lower portion of timber pile below the permanent water table and an upper portion of concrete. In any case, forming proper joints between two dissimilar materials is difficult, and for that reason, composite piles are not widely used Estimating Pile Length Selecting the type of pile to be used and estimating its necessary length are fairly difficult tasks that require good judgment. In addition to being broken down into the classification given in Section 11.2, piles can be divided into three major categories, depending on their lengths and the mechanisms of load transfer to the soil: (a) point bearing piles, (b) friction piles, and (c) compaction piles. Point Bearing Piles If soil-boring records establish the presence of bedrock or rocklike material at a site within a reasonable depth, piles can be extended to the rock surface. (See Figure 11.6a.) In this case, the ultimate capacity of the piles depends entirely on the load-bearing capacity of the underlying material; thus, the piles are called point bearing piles. In most of these cases, the necessary length of the pile can be fairly well established. If, instead of bedrock, a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into the hard stratum. (See Figure 11.6b.) Piles with pedestals can be constructed on the bed of the hard stratum, and the ultimate pile load may be expressed as Q u 5 Q p 1 Q s (11.5) where Q p 5 load carried at the pile point Q s 5 load carried by skin friction developed at the side of the pile (caused by shearing resistance between the soil and the pile) If Q s is very small, Q s < Q p (11.6) In this case, the required pile length may be estimated accurately if proper subsoil exploration records are available.

13 11.3 Estimating Pile Length 547 Q u Q u Q u L Weak soil L Q s Weak soil L Q s Weak soil L b Strong soil layer Q p Rock Q p Q p Q u Q p (a) Q u Q p L b depth of penetration into bearing stratum (b) Q u Q s (c) Figure 11.6 (a) and (b) Point bearing piles; (c) friction piles Friction Piles When no layer of rock or rocklike material is present at a reasonable depth at a site, point bearing piles become very long and uneconomical. In this type of subsoil, piles are driven through the softer material to specified depths. (See Figure 11.6c.) The ultimate load of the piles may be expressed by Eq. (11.5). However, if the value of Q p is relatively small, then Q u < Q s (11.7) These piles are called friction piles, because most of their resistance is derived from skin friction. However, the term friction pile, although used often in the literature, is a misnomer: In clayey soils, the resistance to applied load is also caused by adhesion. The lengths of friction piles depend on the shear strength of the soil, the applied load, and the pile size. To determine the necessary lengths of these piles, an engineer needs a good understanding of soil pile interaction, good judgment, and experience. Theoretical procedures for calculating the load-bearing capacity of piles are presented later in the chapter. Compaction Piles Under certain circumstances, piles are driven in granular soils to achieve proper compaction of soil close to the ground surface. These piles are called compaction piles. The lengths of compaction piles depend on factors such as (a) the relative density of the soil before compaction, (b) the desired relative density of the soil after compaction, and (c) the required depth of compaction. These piles are generally short; however, some field tests are necessary to determine a reasonable length.

14 548 Chapter 11: Pile Foundations 11.4 Installation of Piles Most piles are driven into the ground by means of hammers or vibratory drivers. In special circumstances, piles can also be inserted by jetting or partial augering. The types of hammer used for pile driving include (a) the drop hammer, (b) the single-acting air or steam hammer, (c) the double-acting and differential air or steam hammer, and (d) the diesel hammer. In the driving operation, a cap is attached to the top of the pile. A cushion may be used between the pile and the cap. The cushion has the effect of reducing the impact force and spreading it over a longer time; however, the use of the cushion is optional. A hammer cushion is placed on the pile cap. The hammer drops on the cushion. Figure 11.7 illustrates various hammers. A drop hammer (see Figure 11.7a) is raised by a winch and allowed to drop from a certain height H. It is the oldest type of hammer used for pile driving. The main disadvantage of the drop hammer is its slow rate of blows. The principle of the single-acting air or steam hammer is shown in Figure 11.7b. The striking part, or ram, is raised by air or steam pressure and then drops by gravity. Figure 11.7c shows the operation of the double-acting and differential air or steam hammer. Air or steam is used both to raise the ram and to push it downward, thereby increasing the impact velocity of the ram. The diesel hammer (see Figure 11.7d) consists essentially of a ram, an anvil block, and a fuel-injection system. First the ram is raised and fuel is injected near the anvil. Then the ram is released. When the ram drops, it compresses the air fuel mixture, which ignites. This action, in effect, pushes the pile downward and raises the ram. Diesel hammers work well under hard driving conditions. In soft soils, the downward movement of the pile is rather large, and the upward movement of the ram is small. This differential may Exhaust Cylinder Intake Ram Ram Hammer cushion Pile cap Pile cushion Pile Hammer cushion Pile cap Pile cushion Pile (a) (b) Figure 11.7 Pile-driving equipment: (a) drop hammer; (b) single-acting air or steam hammer

15 11.4 Installation of Piles 549 Exhaust and Intake Cylinder Exhaust and Intake Ram Ram Hammer cushion Pile cap Pile cushion Pile Anvil Pile cushion Pile cap Hammer cushion Pile (c) (d) Static weight Oscillator Clamp Pile (e) (f) Figure 11.7 (continued) Pile-driving equipment: (c) double-acting and differential air or steam hammer; (d) diesel hammer; (e) vibratory pile driver; (f) photograph of a vibratory pile driver (Courtesy of Michael W. O Neill, University of Houston)

16 550 Chapter 11: Pile Foundations Table 11.4 Examples of Commercially Available Pile-Driving Hammers Maker of Model Rated energy Ram weight hammer No. Hammer type kn-m Blows/min kn V 400C Single acting M S M S M S R 5/O R 2/O V 200C Double acting V 140C or V 80C differential V 65C R 150C V 4N100 Diesel V IN M DE M DE V Vulcan Iron Works, Florida M McKiernan-Terry, New Jersey R Raymond International, Inc., Texas not be sufficient to ignite the air fuel system, so the ram may have to be lifted manually. Table 11.4 provides some examples of commercially available pile-driving hammers. The principles of operation of a vibratory pile driver are shown in Figure 11.7e. This driver consists essentially of two counterrotating weights. The horizontal components of the centrifugal force generated as a result of rotating masses cancel each other. As a result, a sinusoidal dynamic vertical force is produced on the pile and helps drive the pile downward. Figure 11.7f is a photograph of a vibratory pile driver. Figure 11.8 shows a pile-driving operation in the field. Jetting is a technique that is sometimes used in pile driving when the pile needs to penetrate a thin layer of hard soil (such as sand and gravel) overlying a layer of softer soil. In this technique, water is discharged at the pile point by means of a pipe 50 to 75 mm in diameter to wash and loosen the sand and gravel. Piles driven at an angle to the vertical, typically 14 to 20, are referred to as batter piles. Batter piles are used in group piles when higher lateral load-bearing capacity is required. Piles also may be advanced by partial augering, with power augers (see Chapter 2) used to predrill holes part of the way. The piles can then be inserted into the holes and driven to the desired depth. Piles may be divided into two categories based on the nature of their placement: displacement piles and nondisplacement piles. Driven piles are displacement piles, because they move some soil laterally; hence, there is a tendency for densification of soil surrounding them. Concrete piles and closed-ended pipe piles are high-displacement piles. However, steel H-piles displace less soil laterally during driving, so they are lowdisplacement piles. In contrast, bored piles are nondisplacement piles because their placement causes very little change in the state of stress in the soil.

17 11.5 Load Transfer Mechanism 551 Figure 11.8 A pile-driving operation in the field (Courtesy of E. C. Shin, University of Incheon, Korea) 11.5 Load Transfer Mechanism The load transfer mechanism from a pile to the soil is complicated. To understand it, consider a pile of length L, as shown in Figure 11.9a. The load on the pile is gradually increased from zero to at the ground surface. Part of this load will be resisted by Q (z50)

18 552 Chapter 11: Pile Foundations Q u Q (z 0) Q (z 0) Q z z L Q 1 Q (z) Q (z) 1 2 Q 2 Q 2 Q p Q s (a) (b) z 0 Unit frictional resistance f (z) Q (z) p z Q u L Q s Zone II Pile tip Zone I Zone II z L (c) Q p (d) (e) Figure 11.9 Load transfer mechanism for piles the side friction developed along the shaft, Q 1, and part by the soil below the tip of the pile, Q 2. Now, how are Q 1 and Q 2 related to the total load? If measurements are made to obtain the load carried by the pile shaft, Q (z), at any depth z, the nature of the variation found will be like that shown in curve 1 of Figure 11.9b. The frictional resistance per unit area at any depth z may be determined as f (z) 5 DQ (z) (p)(dz) (11.8)

19 11.5 Load Transfer Mechanism 553 where p 5 perimeter of the cross section of the pile. Figure 11.9c shows the variation of f (z) with depth. If the load Q at the ground surface is gradually increased, maximum frictional resistance along the pile shaft will be fully mobilized when the relative displacement between the soil and the pile is about 5 to 10 mm, irrespective of the pile size and length L. However, the maximum point resistance Q 2 5 Q p will not be mobilized until the tip of the pile has moved about 10 to 25% of the pile width (or diameter). (The lower limit applies to driven piles and the upper limit to bored piles). At ultimate load (Figure 11.9d and curve 2 in Figure 11.9b), Q (z50) 5 Q u. Thus, and The preceding explanation indicates that (or the unit skin friction, f, along the pile shaft) is developed at a much smaller pile displacement compared with the point resistance, Q p. In order to demonstrate this point, let us consider the results of a pile load test conducted in the field by Mansur and Hunter (1970). The details of the pile and subsoil conditions are as follow: Type of pile: Steel pile with 406 mm outside diameter with 8.15 mm wall thickness Type of subsoil: Sand Length of pile embedment: 16.8 m Figure 11.10a shows the load test results, which is a plot of load at the top of the pile 3Q (z50) 4 versus settlement(s). Figure 11.10b shows the plot of the load carried by the pile shaft 3Q (z) 4 at any depth. It was reported by Mansur and Hunter (1970) that, for this test, at failure Q u < 1601 kn and Q 1 5 Q s Q 2 5 Q p Q s Q p < 416 kn Q s < 1185 kn Now, let us consider the load distribution in Figure 11.10b when the pile settlement(s) is about 2.5 mm. For this condition, Q (z50) < 667 kn Q 2 < 93 kn Q 1 < 574 kn Hence, at s mm, Q (100) % Q p 416 and Q (100) % Q s 1185 Thus, it is obvious that the skin friction is mobilized faster at low settlement levels as compared to the point load.

20 554 Chapter 11: Pile Foundations Load at the top of pile, Q (z = 0) (kn) Q (z = 0) (kn) s = 2.5 mm s = 5 mm Settlement, s (mm) Depth, z (m) 6 9 s = 11 mm (a) 16.8 (b) Figure Load test results on a pipe pile in sand (Based on Mansur and Hunter, 1970) At ultimate load, the failure surface in the soil at the pile tip (a bearing capacity failure caused by Q p ) is like that shown in Figure 11.9e. Note that pile foundations are deep foundations and that the soil fails mostly in a punching mode, as illustrated previously in Figures 3.1c and 3.3. That is, a triangular zone, I, is developed at the pile tip, which is pushed downward without producing any other visible slip surface. In dense sands and stiff clayey soils, a radial shear zone, II, may partially develop. Hence, the load displacement curves of piles will resemble those shown in Figure 3.1c Equations for Estimating Pile Capacity The ultimate load-carrying capacity where Q u of a pile is given by the equation Q u 5 Q p 1 Q s Q p 5 load-carrying capacity of the pile point Q s 5 frictional resistance (skin friction) derived from the soil pile interface (see Figure 11.11) (11.9) Numerous published studies cover the determination of the values of Q p and Q s. Excellent reviews of many of these investigations have been provided by Vesic (1977), Meyerhof (1976), and Coyle and Castello (1981). These studies afford an insight into the problem of determining the ultimate pile capacity.

21 11.6 Equations for Estimating Pile Capacity 555 Q u Steel Soil plug L L b Q s D (b) Open-Ended Pipe Pile Section D q Steel d 1 Soil plug Q p L length of embedment L b length of embedment in bearing stratum (a) d 2 (c) H-Pile Section (Note: A p area of steel soil plug) Figure Ultimate load-carrying capacity of pile Point Bearing Capacity, The ultimate bearing capacity of shallow foundations was discussed in Chapter 3. According to Terzaghi s equations, and Q p q u 5 1.3crN c 1 qn q 1 0.4gBN g q u 5 1.3crN c 1 qn q 1 0.3gBN g Similarly, the general bearing capacity equation for shallow foundations was given in Chapter 3 (for vertical loading) as q u 5 crn c F cs F cd 1 qn q F qs F qd 1 1 2gBN g F gs F gd Hence, in general, the ultimate load-bearing capacity may be expressed as q u 5 crn c * 1 qn q * 1gBN g * (11.10) where N c *, N q *, and N g * are the bearing capacity factors that include the necessary shape and depth factors. Pile foundations are deep. However, the ultimate resistance per unit area developed at the pile tip, q p, may be expressed by an equation similar in form to Eq. (11.10), although the values of N c *, N q *, and N g * will change. The notation used in this chapter for the width of a pile is D. Hence, substituting D for B in Eq. (11.10) gives q u 5 q p 5 crn c * 1 qn q * 1gDN g * (for shallow square foundations) (for shallow circular foundations) (11.11)

22 556 Chapter 11: Pile Foundations Because the width D of a pile is relatively small, the term gdn g * may be dropped from the right side of the preceding equation without introducing a serious error; thus, we have q p 5 crn c * 1 qrn q * (11.12) Note that the term q has been replaced by qr in Eq. (11.12), to signify effective vertical stress. Thus, the point bearing of piles is Q p 5 A p q p 5 A p (crn c * 1 qrn q *) (11.13) where A p 5 area of pile tip c r 5 cohesion of the soil supporting the pile tip q p 5 unit point resistance q r 5 effective vertical stress at the level of the pile tip N c *, N q * 5 the bearing capacity factors Frictional Resistance, Q s The frictional, or skin, resistance of a pile may be written as Q s 5S p DLf (11.14) where p 5 perimeter of the pile section DL 5 incremental pile length over which p and f are taken to be constant f 5 unit friction resistance at any depth z The various methods for estimating Q p and Q s are discussed in the next several sections. It needs to be reemphasized that, in the field, for full mobilization of the point resistance (Q p ), the pile tip must go through a displacement of 10 to 25% of the pile width (or diameter). Allowable Load, Q all After the total ultimate load-carrying capacity of a pile has been determined by summing the point bearing capacity and the frictional (or skin) resistance, a reasonable factor of safety should be used to obtain the total allowable load for each pile, or Q all 5 Q u FS where Q all 5 allowable load-carrying capacity for each pile FS 5 factor of safety The factor of safety generally used ranges from 2.5 to 4, depending on the uncertainties surrounding the calculation of ultimate load.

23 11.7 Meyerhof s Method for Estimating 11.7 Meyerhof s Method for Estimating Q p 557 Q p Sand The point bearing capacity, q p, of a pile in sand generally increases with the depth of embedment in the bearing stratum and reaches a maximum value at an embedment ratio of L b >D 5 (L b >D) cr. Note that in a homogeneous soil L b is equal to the actual embedment length of the pile, L. However, where a pile has penetrated into a bearing stratum, L b, L. Beyond the critical embedment ratio, (L b >D) cr, the value of q p remains constant (q p 5 q l ). That is, as shown in Figure for the case of a homogeneous soil, L 5 L b. For piles in sand, cr 5 0, and Eq. (11.13) simpifies to Q p 5 A p q p 5 A p qrn* q (11.15) The variation of N q * with soil friction angle fr is shown in Figure The interpolated values of N q * for various friction angles are also given in Table However, Q p should not exceed the limiting value A p q l ; that is, Q p 5 A p qrn q * < A p q l (11.16) Figure Variation of the maximum values of N q * with soil friction angle fr (From Meyerhof, G. G. (1976). Bearing Capacity and Settlement of Pile Foundations, Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 102, No. GT3, pp With permission from ASCE.) (L b /D) cr Unit point resistance, q p N* q N* q q p q l 2 L/D L b /D Figure Nature of variation of unit point resistance in a homogeneous sand Soil friction angle, (deg)

24 558 Chapter 11: Pile Foundations Table 11.5 Interpolated Values of N q * Based on Meyerhof s Theory Soil friction angle, f (deg) N q * The limiting point resistance is q l p a N q * tan fr (11.17) where p pressure ( 5100 kn>m 2 a 5 atmospheric ) fr 5 effective soil friction angle of the bearing stratum A good example of the concept of the critical embedment ratio can be found from the field load tests on a pile in sand at the Ogeechee River site reported by Vesic (1970). The pile tested was a steel pile with a diameter of 457 mm. Table 11.6 shows the ultimate resistance at various depths. Figure shows the plot of q p with depth obtained from the field tests along with the range of standard penetration resistance at the site. From the figure, the following observations can be made. 1. There is a limiting value of q p. For the tests under consideration, it is about 12,000 kn/m The ( L>D) cr value is about 16 to 18.

25 11.7 Meyerhof s Method for Estimating Q p 559 Table 11.6 Ultimate Point Resistance, q p, of Test Pile at the Ogeechee River Site As reported by Vesic (1970) Pile diameter, Depth of embedment, q p D (m) L (m) L,D ( kn,m 2 ) , , , , ,971 0 Pile point resistance, q p (kn/m 2 ) ,000 16,000 20, Pile point resistance Depth (m) Range of N 60 at site N 60 Figure Vesic s pile test (1970) result variation of q p and N 60 with depth 3. The average N 60 value is about 30 for L>D > (L>D) cr. Using Eq. (11.37), the limiting point resistance is 4p a N 60 5 (4)(100)(30) 5 12,000 kn/m 2. This value is generally consistent with the field observation. Clay ( f50) For piles in saturated clays under undrained conditions (f 50), the net ultimate load can be given as Q p < N c *c u A p 5 9c u A p (11.18) where c u 5 undrained cohesion of the soil below the tip of the pile.

26 560 Chapter 11: Pile Foundations 11.8 Vesic s Method for Estimating Sand Q p Vesic (1977) proposed a method for estimating the pile point bearing capacity based on the theory of expansion of cavities. According to this theory, on the basis of effective stress parameters, we may write Q p 5 A p q p 5 A p s o r N s * (11.19) where sr o 5 mean effective normal ground stress at the level of the pile point and K o qr 3 K o 5 earth pressure coefficient at rest sin fr N s * 5 bearing capacity factor Note that Eq. (11.19) is a modification of Eq. (11.15) with (11.20) (11.21) N s * 5 3N q * (1 1 2K o ) (11.22) According to Vesic s theory, N s * 5 f(i rr ) where I rr 5 reduced rigidity index for the soil. However, (11.23) where I rr 5 I r 1 1 I r D (11.24) E s I r 5 rigidity index 5 2(1 1m s ) qr tan fr 5 G s qr tan fr E s 5 modulus of elasticity of soil m s 5 Poisson s ratio of soil G s 5 shear modulus of soil D5average volumatic strain in the plastic zone below the pile point The general ranges of I r for various soils are (11.25) Sand(relative density 5 50% to 80%): 75 to 150 Silt : 50 to 75 In order to estimate I r [Eq. (11.25)] and hence I rr [Eq. (11.24)], the following approximations may be used (Chen and Kulhawy, 1994)

27 11.8 Vesic s Method for Estimating Q p 561 E s p a 5 m (11.26) where p a 5 atmospheric pressure ( < 100 kn>m 2 ) 100 to 200(loose soil) m 5 c200 to 500(medium dense soil) 500 to 1000(dense soil) fr 2 25 m s a b (for 25 #fr # 45 ) 20 (11.27) D50.005a1 2 fr b qr p a (11.28) On the basis of cone penetration tests in the field, Baldi et al. (1981) gave the following correlations for I r : and I r F r (%) I r F r (%) (for mechanical cone penetration) (11.29) (for electric cone penetration) (11.30) For the definition of F r, see Eq. (2.41). Table 11.7 gives the values of N s * for various values of I rr and fr. Clay (f 50) In saturated clay ( f50 condition), the net ultimate point bearing capacity of a pile can be approximated as Q p 5 A p q p 5 A p c u N c * (11.31) where c u 5 undrained cohesion According to the expansion of cavity theory of Vesic (1977), N c * (ln I rr 1 1) 1 p (11.32) The variations of N c * with I rr for f50condition are given in Table Now, referring to Eq. (11.24) for saturated clay with no volume change, D50. Hence, I rr 5 I r (11.33)

28 562 Table 11.7 Bearing Capacity Factors N s * Based on the Theory of Expansion of Cavities I rr f From Design of Pile Foundations, by A. S. Vesic. SYNTHESIS OF HIGHWAY PRACTICE by AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORT. Copyright 1969 by TRANSPORTATION RESEARCH BOARD. Reproduced with permission of TRANSPORTATION RESEARCH BOARD in the format Textbook via Copyright Clearance Center.

29 11.9 Coyle and Castello s Method for Estimating Q p in Sand 563 Table 11.8 Variation of N c * with I rr for f50condition based on Vesic s Theory I rr N c * For f50, I r 5 E s 3c u (11.34) O Neill and Reese (1999) suggested the following approximate relationships for I r and the undrained cohesion, c u. c u p a I r Note: p a atmospheric pressure < 100 kn>m 2. The preceding values can be approximated as I r 5 347a c u p a b 2 33 # 300 (11.35) 11.9 Coyle and Castello s Method for Estimating in Sand Coyle and Castello (1981) analyzed 24 large-scale field load tests of driven piles in sand. On the basis of the test results, they suggested that, in sand, where Q p 5 qrn q *A p q r 5 effective vertical stress at the pile tip N q * 5 bearing capacity factor Q p Figure shows the variation of N q * with L>D and the soil friction angle fr. (11.36)

30 564 Chapter 11: Pile Foundations Bearing capacity factor, N * q Embedment ratio, L/D Figure Variation of N q * with L>D (Redrawn after Coyle and Costello, 1981) Example 11.1 Consider a 15-m long concrete pile with a cross section of 0.45 m m fully embedded in sand. For the sand, given: unit weight, g517kn>m 3 ; and soil friction angle, fr Estimate the ultimate point with each of the following: a. Meyerhof s method b. Vesic s method c. The method of Coyle and Castello d. based on the results of parts a, b, and c, adopt a value for Solution Part a From Eqs. (11.16) and (11.17), For fr 5 35, the value of N q * < 143 (Table 11.5). Also, qr 5gL 5 (17)(15) 5 255kN>m 2. Thus, A p qrn q * 5 ( )(255)(143) < 7384kN Again, A p (0.5p a N q * tan fr) 5 ( )3(0.5)(100)(143)( tan 35)4 < 1014 kn Hence, Q p kn. Part b From Eq. (11.19), Q p Q p 5 A p qrn q * # A p (0.5p a N q * tan fr) Q p

31 11.9 Coyle and Castello s Method for Estimating Q p in Sand 565 Q p 5 A p s o rn s * s o r 5 c From Eq. (11.26), Assume m < 250 (medium sand). So, From Eq. (11.27), From Eq. (11.28), D50.005a (1 2 sin fr) 1 1 2(1 2 sin 35) d qr 5 a b ( ) 3 3 From Eq. (11.25), I r 5 From Eq. (11.24), From Table 11.7, for fr 5 35 and I rr , the value of N s * < 55. Hence, Part c From Eq. (11.36), fr E s 2(1 1m s )qr tan fr I rr kN>m 2 fr 2 25 m s a b a 20 ba qr p a b a1 2 5 E s p a 5 m E s 5 (250)(100) 5 25,000 kn>m 2 25,000 (2)( )( )( tan 35) 5 56 I r 1 1 I r D (56)(0.0064) Q p 5 A p s o rn s * 5 ( )(139.96)(55) < 1559 kn Q p 5 qrn q *A p L D b ba b For fr 5 35 and L>D , the value of N q * is about 48 (Figure 11.15). Thus, Q p 5 qrn q *A p 5 ( )(48)( ) < 2479 kn Part d It appears that Q p obtained from the method of Coyle and Castello is too large. Thus, the average of the results from parts a and b is

32 566 Chapter 11: Pile Foundations kn Use Q p kn. Example 11.2 Consider a pipe pile (flat driving point see Figure 11.2d) having an outside diameter of 406 mm. The embedded length of the pile in layered saturated clay is 30 m. The following are the details of the subsoil: Depth from ground surface (m) Saturated unit weight, g(kn>m 3 ) The groundwater table is located at a depth of 5 m from the ground surface. Estimate by using Q p a. Meyerhof s method b. Vesic s method Solution Part a From Eq. (11.18), The tip of the pile is resting on a clay with c u kn>m 2. So, Part b From Eq. (11.31), From Eq. (11.35), c u (kn>m 2 ) Q p 5 9c u A p Q p 5 (9)(100) ca p ba b d kn Q p 5 A p c u N c * I r 5 I rr 5 347a c u b a 100 b p a 100 So we use I rr From Table 11.8 for I rr 5 300, the value of N c * Thus, Q p 5 A p c u N c * 5 ca p ba b d (100)(11.51) kn

33 11.10 Correlations for Calculating Q p with SPT and CPT Results 567 Note: The average value of Q p is < 133 kn Correlations for Calculating Q p with SPT and CPT Results On the basis of field observations, Meyerhof (1976) also suggested that the ultimate point resistance q p in a homogeneous granular soil (L 5 L b ) may be obtained from standard penetration numbers as q p 5 0.4p a N L 60 (11.37) D # 4p an 60 where N 60 5 the average value of the standard penetration number near the pile point (about 10D above and 4D below the pile point) p a 5 atmospheric pressure ( < 100 kn>m 2 or 2000 lb>ft 2 ) Briaud et al. (1985) suggested the following correlation for the standard penetration resistance. N 60 q p p a (N 60 ) 0.36 Meyerhof (1956) also suggested that q p < q c (in granular soil) where q c 5 cone penetration resistance. q p in granular soil with (11.38) (11.39) Example 11.3 Consider a concrete pile that is m m in cross section in sand. The pile is 15.2 m long. The following are the variations of with depth. N 60 Depth below ground surface (m) N

34 568 Chapter 11: Pile Foundations Q p Q p a. Estimate using Eq. (11.37). b. Estimate using Eq. (11.38). Solution Part a The tip of the pile is 15.2 m below the ground surface. For the pile, average of 10D above and about 5D below the pile tip is N 60 From Eq. (11.37) N < 24 Q p 5 A p (q p ) 5 A p c0.4p a N 60 a L D bd # A p(4p a N 60 ) D m. The A p c0.4p a N 60 a L 15.2 bd 5 ( ) c (0.4)(100)(24) a bd kn D Thus, Q p kn Part b From Eq. (11.38), A p (4p a N 60 ) 5 ( )3(4)(100)(24) kn Q p 5 A p q p 5 A p 319.7p a (N 60 ) ( )3(19.7)(100)(24) kn Frictional Resistance (Q s ) in Sand According to Eq. (11.14), the frictional resistance Q s 5Sp DLf The unit frictional resistance, f, is hard to estimate. In making an estimation of f, several important factors must be kept in mind: 1. The nature of the pile installation. For driven piles in sand, the vibration caused during pile driving helps densify the soil around the pile. The zone of sand densification may be as much as 2.5 times the pile diameter, in the sand surrounding the pile. 2. It has been observed that the nature of variation of f in the field is approximately as shown in Figure The unit skin friction increases with depth more or

35 11.11 Frictional Resistance (Q s ) in Sand 569 Unit frictional resistance, f z L D f K o L L (a) Depth Figure Unit frictional resistance for piles in sand (b) less linearly to a depth of Lr and remains constant thereafter. The magnitude of the critical depth Lr may be 15 to 20 pile diameters. A conservative estimate would be Lr < 15D (11.40) 3. At similar depths, the unit skin friction in loose sand is higher for a highdisplacement pile, compared with a low-displacement pile. 4. At similar depths, bored, or jetted, piles will have a lower unit skin friction compared with driven piles. Taking into account the preceding factors, we can give the following approximate relationship for f (see Figure 11.16): For z 5 0 to Lr, f 5 Ks o rtan dr (11.41) and for z 5 Lr to L, f 5 f z5lr (11.42) In these equations, K 5 effective earth pressure coefficient s o r 5 effective vertical stress at the depth under consideration dr 5 soil-pile friction angle In reality, the magnitude of K varies with depth; it is approximately equal to the Rankine passive earth pressure coefficient, K p, at the top of the pile and may be less than